Denison, William C. 1979. Lobaria oregana, a nitrogen-fixing lichen in old-growth Douglas-fir forests. In: Gordon, J. C.; Wheeler, C. T.; Perry, D. A., eds. Sympiotic nitrogen fixation in the management of temperate forests: proceedings of a workshop; 1979 April 2-5; Corvallis, OR. Corvallis, OR: Forest Research Laboratory, Oregon State University: 266-275.
Lobaria oregana (Tuck.) Mnl. Arg. occurs on the branches of old-growth Douglas firs in amounts estimated at 10-15 kg dry weightper tree, or approximately 500 kg/ha. This is roughly 5% of theweight of foliage on the Douglas firs. Other nitrogen-fixinglichens occur in much smaller quantities; usually less than 1% ofthe weight of L. oregana. We found no nitrogen-fixing bacteria orblue green algae on the surfaces of foliage or twigs.
Two methods gave estimates of nitrogen fixed by L. oregana in therange 3-4 kg/ha/yr. Annual growth of the lichen, measured bysequential photographs, averaged 30%, or 150 kg/ha/yr. Thenitrogen content of the lichen is 2.1%, amounting to 3.15 kg/haof nitrogen in each year's new growth. Assuming that all of thenitrogen required for growth is fixed by the lichen, this is arough estimate of annual fixation. Another method used theacetylene reduction technique to estimate rates of fixation. Theaverage rate of fixation during the wet season (15 September -15 June) when the lichens are active was 78 nanomoles of nitrogenper gram of dry lichen per hour. Assuming that all fixationoccurs during the wet season this amounts to 3.5 kg/ha/yr.
The principal factors influencing the rate of fixation were themoisture content of the lichen and the temperature. Light onlyaffects the rate indirectly. Fixation continues undiminished in the dark, presumably until stored food is exhausted. Transferof nitrogen from L. oregana to the forest floor occurs by litter-fall and decomposition and by leaching from intact lichens.Grazing by invertebrates could not be demonstrated.